Multistage amplifier



' July 11, 1933. P, BORIAS 1,917,426

- MULT I STAGE AMPLIFIER Filed Feb. 17, 1928 2 Sheets-Sheet 1 INVENTOR PUL B RAS [Zia/M AT RNEY July 11, 1933.. P BORIAS 1,917,426

MULT I STAGE AMPLIFIER Filed Feb. 17, 1928 2 Sheets-Sheet 2 Jig. 7 12;.a

INVENTOR PAUL BORlAS A ORNEY Patented July 11, 1933 UNITED STATES PATENTOFFICE PAUL toxins or rams, FRANCE, ASSIGNOR T socrnrn FRANGAISE RADIOErna TRIQUE, or PARIS, rnnncn, A JOINT-STOCK COMPANY or FRANCE'MULTISTAGE AMPLIFIER- .Apfalication filed. February 17, 1928, SerialNo. 254,988, and. in France MarchB, 1927.

,The present invention relates to new methods of and arrangements forcoupling multielectrode tubes, and more particularly to radio frequencyamplifiers. As contrasted with arrangements known in the prior art, itoffers the following ad? vantages: absence of reaction due to theinternal capacitance of the tubes, considerably higheramplification (atthe same damping), possibility of choosing the form of the resonantcoupling so as to insure a filtering effect, regulation of selectivityindependently of the amplification.

The underlying principle of the invention is as follows: whereas in thearrangements known in practice, one starts from a high frequencypotential available at the plate and feeds it to the following grideither directly (resistance, choke-coil, tuned platecircuit schemes) orindirectly by stepping it up by a tuned or non-tuned transformer(transformer coupling), according to the present invention the rcactanceof the'plate circuit for the, frequency to be received is eliminated,with the result that all reaction due to the internal capacitance of thetube is prevented, inasmuch as the plate is at an alternating currentpotential practically of zero value.

The invention, both as to its construction and mode of operationtogether with the objects and advantages thereof, willbest be understoodby reference to the following description taken in connection with theac-. companying drawings, of which Figures 1-12 illustratediagrammatically various forms of the invention. In all the figuressimilar reference characters denote similar or equiva lent elements. 1

The simplest embodiment according to the present disclosure is shown byway of example in Figure 1. Referring to the illus tration, 1 and 2denote two tubes'to be connected cascade-fashion, here assumed to be ofthe triode type, and comprising the three elements, to wit: filament,grid and plate of the' usual kind: Twoimpedances, L and C, adjustable ifdesired, are connected in series, and the circuit element thusconstititted is connected between the plate and the filament of tube 1.The grid of tube 2 is connected between the two impedances abovementioned. For the working fre-' quency the reactance of impedance L iscompensated by the reactance of the impedance C. To apply to the plateof the tube the necessary direct current potential, the said andcapacitance, though it will be understood that, if desired, they couldbe transposed or one substituted for the other, or bothbe replaced bycomplex assemblies of capacitive ,and inductive elements, provided thatthe resultant reactances' of each assembly become compensated ashas beenpointed out.

Figure 2 shows an arrangement according to this invention which is amodification of the scheme shown in Figure 1 by the transposition orexchange of the inductive and capacitive reactances. Inthis case acoupling capacity 0 is inserted in the lead brought to the grid of tube2, while the potential of the latter with reference to the filament isdetermined and governed, for instance, by means of a grid-leakresistance R. It will be noted that, with reference to. the resonantfrequency, the arrangement of F igure 1 leaves a residue ofamplification for the higher frequencies, but that it does not transferthe lower frequencies, whereas the scheme Figure 2 leaves a residue ofamplification for the lower frequencies, while it does not transmitthe-higher frequencies.

The modification shown in Figure 3 has recourse to an inductive couplingfor the application of the potential to the grid of tube 2, and asa'result it allows of raising the po'-' .tential impressed upon the gridof tube 2.

Application of the variable potential to the grid of tube 2 is insuredby replacing the direct connection of this grid with impedances L and Oof the scheme Figure 1 by the magnetic induction of coil L upon a coil Linserted in the circuit of the grid of tube 2,. the said coil L playingthe part of the secondary of a transformer with an incidentalstepping-up of the potential.

Primarily in the case of amplifier arrangements working at a fixedfrequency, or frequencies varying only inside a narrow band, it isequally well to use, in lieu and stead of a capacity, a self-inductancecoil which could be operated on a shorter wave than its .own (natural)period and which, for this reason, would behave like a capacity. In thiscase, there would thus be two self-inductances in series, and they wouldhave to be proportioned in sucha way that operation takes place abovethe shortest natural wave of an inductive reactance, and below that ofthe longest wave corresponding to a capacitive reactance. The advantageresiding in this plan isthat both the very low and the very'highfrequencies with reference to the resonant frequency are precluded. 7Next, for the natural period of one of the two coils, there will occur avery appreciable reduction in amplification, especially in the case ofthe one inserted between the plate and'the grid of the tube'next above.This deformation of the resonance'curve may be put to practicaladvantage for the purpose of providing a sort of band-filter byarranging in two successive stages the large self-inductance coil, bothoutside and inside the grid circuit of the second tube.

1 One particular embodiment of the modification employing two coils inseries which is of particularly great'advantage, consists in using'twoidentical-coils, the one intend-- ed to operate as a capacity beingshunted by a condenser, the same being tuned to a fre quency close toF-, where F is the desired signal frequency to be received. For theworking frequency F p the stopper thus formed possesses a reactancecompensating that of the other coil, and an apparent resistance equal tothe ohmic resistance of the coil. Arrangements of this kind comprisingcoils give amplifications higher than those obtainable by the precedingschemes (Figures 1 to 3). They are schematically illus trated in Figures4 and 5 where the same symbols designate the same elements as heretoforeso thatno further explanations will be required. It will be noted thatthese coil arrangements makeit possible to dispense with the impedance Zin the preceding figures, the potential of the direct current sourcebeing applied to the plate bythe coil circuit directly. Figure & is ascheme equivalent to Figure 1, and Figure 5 the equivalent to Figure 2,looked at from the viewpoint of arrangement respectively of theinductance andcapacitive reactances with relationeto the plate of thefirst tube and the connection brought to the grid of the second tube.

One of the properties of the coupling system disclosed in thisinvention, especially when it comprises a resistance Z for directcurrent supply, is that it is possible to -regulate the degree ofselectance independently of the amplification, contrary to What is trueof resonant or reaction coupling. For, in-

stance, when referring to Fig. 1, the'sharp ness of the resonancecurvedepends upon the overvoltage across the terminals of the self-inductancecoil. Now, the resistancewhich is in series with the self-inductancecoil and the capacity includes the internal impedance of the tube andthe supply impedance Z which may be reduced to a resistance. Thevariations of the resistance Z do not influence amplification undertuned condition, though it is possible to graduate selectivity.

An improved modification of the basic idea of the invention whereby itis possible to benefit more largely from-this property consists in theuse of a two-grid tube or tetrode, of the type in which two grids orauxiliary electrodes are provided at different distances betweenfilament and plate. Figure 6 by way of example shows one form ofconstruction of this scheme. The oscillations to be amplified areimpressed upon the inner grid of the double-grid tube 1,,the outer gridbeing raised to a certain positive potential by being connected, forexample, to an intermediate point of the plate battery B.

The remainder of the circuit elements of the 7 plate and of tube 2 isidentical with the-denotations above given for the case of the triodes(see, for instance, Figures 1 to 3). This arrangement allows of adoptinga resistance Z of high value, and therefore of securing a veryfiat-topped resonance curve. Moreover, the outer gridi united with theplate battery constitutes an electrostatic shield between the controlgrid and the plate, with a further reduction of the chances of reaction.It will be understood, of course, that in the scheme shown in Figure 6,as indicated for the preceding schemes, it is possible to transpose thecapacitive impedances O and th inductive impedances L, or to -em-. ploydivers or complex elements (coils operating above or below their naturalperiods, coils with capacitance across their terminals, etc.) In Figure7 is shown an arrangement similar to Figure 6 with the outer grid actingas the control grid in the input circuit and the inner grid beingsupplied with posi? tive potential from the plate battery.

Finally, although the amplification figures attainable by the aid of thepresent disclosures are far higher than those obtained by theconventional forms of arrangement, whereby it is made possible to designefficient amplifiers free from retro-action, it will be noted that theinvention is particularly suited to the use of regeneration. Indeed, bythe addition of regenerative couplings, it is feasible to insureconsiderable amplification figures, far higher, at any rate, than thoseobtainable in the usual regenerative circuit arrangements, in thepresence of equal clamping. Moreover, the use of regeneration does notincrease the chances of self-excitation in the schemes according to thepresent invention, whereas the dangers from this source are quiteconsiderable in the customary schemes.

The adoption of regenerative coupling in circuit arrangements accordingto this invention, as will be understood, is capable of a great nmnberof modifications. All that is necessary with that end in view is to takeoff at a convenient point of the circuits a portion of the amplifiedpotential having convenient size and phase, and to apply the same to theinput circuit of one of the amplifier tubes. The necessary conditions ashereinbefore set forth according to this invention are realized byadopting the following scheme:

The potential to be fed back, if taken off simply at one of the couplingimpedances (say, inductance coil L of the arrangements illustrated inFigures 1, 2 and 3) must be fed back not to the grid of the tube inWhose plate circuit the impedance in question is included, but rather tothe grid of the tube next below supposed to be coupled with the tube inquestion in one of the ways adapted to preserve phase coincidencebetween the grid potential and the plate current of the tube (forinstance, resistance-coupling arrangements or arrangements according tothis invention).

It is generally preferable to practice reaction in the same tube. Forthis object, one efficient scheme consists in adopting one of thearrangements of this invention comprising a complex impedance(self-inductance and capacity in parallel), to insert in the branchcontaining the self-inductance coil a series resistance r, and to feedback to the grid of the particular tube in whose plate circuit isincluded the said impedance, the potential taken off across theterminals of the said resistance, with the interposition, if necessary,of such capacities as may be required so as to prevent theshort-circuiting of the sources, or the application of undesirabledirect current potentials to the grid. Regulation of the feedback may beeffected by the adjustment of the said resistance or else by the settingof the rheostat controlling the heating of the tube.

Figures 8, 9 and 10 of the accompanying drawing illustrate variousembodiments of the scheme before described, there having been chosen, byway of example, the case of duction in the disturbances reaching thereceiver in the shape of aperiodic shocks. This end is realized byemploying as the impedance for the direct current supply of the plate aself-inductance coil in the circuit arrangements shown in Figures 1 to3, the result being arrangements of the kind illustrated in Figures 11and 12. The impulse due to a shock will cause the tube 1 to oscillate ata frequency (corresponding to the constants of the circuit LCL) whichdiffers from that for which maximum amplification is obtained, and inthis manner it is easier to distinguish the signals from the strays.

Although the invention has been stated to I be especially useful for theamplification of radio frequency currents, it will be obvious that it isequally ell applicable to tuned radio frequency amplifiers Having thusdescribed the nature of my invention, I claim:

In combination a vacuum tube having a cathode, inner grid, outer gridand an anode,

an input circuit including the cathode and said outer grid, an outputcircuit comprising the anode, a resistance having a high value, acurrent source and said cathode all in series, means for connecting saidinner grid to said current source, a second vacuum tube having input andoutput circuits, said input circuit including a reactance adapted to actas a coupling device between said first mentioned tube output circuitand said second mentioned tube input circuit, means for connecting thecathode of said first mentioned tube to one end of said coupling deviceand means comprising another reactance for connecting the anode of saidfirst mentioned tube to the other end of said coupling device.

PAUL BORIAS.

